The need for a high-quality factor (Q), low insertion loss tunable filter pervades a wide range of microwave and RE applications, in both the military, e.g., RADAR, communications and electronic intelligence (ELINT), and the commercial fields such as in various communications applications, including cellular. Placing a sharply defined bandpass filter directly at the receiver antenna input will often eliminate various adverse effects resulting from strong interfering signals at frequencies near the desired signal frequency in such applications. Because of the location of the filter at the receiver antenna input, the insertion loss must be very low so as to not degrade the noise figure. In most filter technologies, achieving a low insertion loss requires a corresponding compromise in filter steepness or selectivity. In the present invention, the extremely low loss property of high-temperature superconductor (HTS) filter elements provides an attractive solution, achieving a very low insertion loss yet simultaneously allowing a high selectivity/steepness bandpass definition.
In many applications, particularly where frequency hopping is used, a receiver filter must be tunable to either to select a desired frequency or to trap an interfering signal frequency. Thus, the insertion of a linear tunable filter between the receiver antenna and the first nonlinear element (typically a low-noise amplifier or mixer) in the receiver offers, providing that the insertion loss is very low, substantial advantages in a wide range of RF and microwave systems. For example, in RADAR systems, high amplitude interfering signals, either from “friendly” nearby sources, or from jammers, can desensitize receivers or intermodulate with high-amplitude clutter signal levels to give false target indications. In high-density signal environments, RADAR warning systems frequently become completely unusable.
Both lumped element and distributed element filters suffer from these and other problems. For example, while distributed-element YIG-tuned filters have been used, the high level of insertion loss (usually greater than 10 dB) of suitable YIG filters necessitates their use on a “switch in when absolutely necessary” basis only, as the degradation to noise figure would generally be unacceptable. Lumped element filters also suffer problems. For a lumped element filter to be tunable, the filter requires either a tunable capacitor, or a tunable inductive element. The vast majority of RF tunable lumped element filters have used varactor diodes. Such a design amounts to using a tunable capacitor because varactor diodes, through changing the reverse bias voltage, vary the depletion thickness and hence the PN junction capacitance. While varactors are simple and robust, they have limited quality factors (Q), and suffer from the problem that the linear process that tunes them extends all the way to the signal frequency, so that high-amplitude signals create, through the resulting nonlinearities, undesirable intermodulation products, etc. The same problems of poor Q and high-frequency nonlinearities are anticipated for “tunable materials” such as ferroelectrics.
Consider the case of a conventional varactor diode. In a varactor, the motion of electrons accomplishes the tuning itself. As the reverse bias (Vr) on the junction of the varactor is changed, then in accordance with Poisson's Equation, the width of the PN junction depletion region changes which alters the capacitance of the junction (Cj). Because the tuning mechanism of varactors is electronic, the tuning speed is extremely fast. Unfortunately, this also leads to a serious associated disadvantage: limited dynamic range. Because the Cj (Vr) relationship is nearly instantaneous in response, extending to changes in Vr at the signal frequency itself, and because the signal (frequently in a resonantly magnified form) appears as a component of the junction bias voltage, Vr, the signal itself parametrically modulates the junction capacitance. If the signal amplitude across the varactor is very small in comparison to the dc bias, the effect is not too serious. Unfortunately, for high signal amplitudes, this parametric modulation of the capacitance can produce severe cross-modulation (IM) effects between different signals, as well as harmonic generation and other undesirable effects. While these signal-frequency varactor capacitance variations are the basis of useful devices such as parametric amplifiers, subharmonic oscillators, frequency multipliers, and many other useful microwave circuits, in the signal paths of conventional receivers they are an anathema. This inherent intermodulation or dynamic range problem will presumably extend to “tunable materials,” such as ferroelectrics or other materials in which the change of dielectric constant (εr) with applied electric field (E) is exploited to tune a circuit. As long as the εr (E) relationship applies out to the signal frequency, then the presence of the signal as a component of the E will lead to the same intermodulation problems that the varactors have.
In addition to the intermodulation/dynamic range problems of varactors, these conventional tuning devices also have serious limitations in Q, or tuning selectivity. Because the varactors operate by varying the depletion region width of a P-N junction, this means that at lower reverse biases (higher capacitances), there is a substantial amount of undepleted moderately-doped semiconductor material between the contacts and the junction that offers significant series resistance (Rac) to ac current flow. Since the Q of a varactor of junction capacitance Cj and series resistance Rac at the signal frequency f is given by Q=1/(2 f Cj Rac), this means that the varactor Q values are limited, particularly at higher frequencies. For example, a typical commercial varactor might have Cj=2.35 pF with Rac=1.0 Ω at Vr=−4V, or Cj=1.70 pF with Rac=0.82 Ω at Vr=−10V, corresponding to Q values at f=1.0 GHz of Q=68 at Vr=−4V or Q=114 at Vr=−10V (or f=10.0 GHz values of Q=6.8 and Q=11.4, respectively). Considering that an interesting X-band (f=10 GHz) RADAR application might want a bandwidth of 20 MHz for the full width at half-maximum (FWHM), corresponding to a Q=500 quality factor, we see that available varactors have inadequate Q (too much loss) to meet such requirements. While the mechanisms are different, this will very likely apply to the use of ferroelectrics or other “tunable materials.” A general characteristic of materials which exhibit the field-dependent dielectric constant nonlinearities (that makes them tunable) is that they exhibit substantial values of the imaginary part of the dielectric constant (or equivalently, loss tangent). This makes it unlikely that, as in varactors, these “tunable materials” will be capable of achieving high Q's, particularly at high signal frequencies.
An additional problem with both varactors and “tunable materials” for circuits with high values of Q is that these are basically two-terminal devices; that is, the dc tuning voltage must be applied between the same two electrodes to which the signal voltage is applied. The standard technique is to apply the dc tuning bias through a “bias tee”-like circuit designed to represent a high reactive impedance to the signal frequency to prevent loss of signal power out the bias port (as this would effectively reduce the Q). However, while the design of bias circuits that limit the loss of energy to a percent, or a fraction of a percent, even losses of a fraction of a percent are not nearly good enough for very high Q circuits (e.g., Q's in the 103 to >104 range, as achievable with HTS resonators). It would be much easier to design such very high Q circuits using three-terminal, or preferably 4-terminal (two-port) variable capacitors in which the tuning voltage is applied to a completely different pair of electrodes from those across which the signal voltage is applied (with an inherent high degree of isolation between the signal and bias ports).
One new form of variable capacitor which avoids the problems of varactors or “tunable materials” approaches is the microelectromechanical (MEMS) variable capacitor such as that disclosed by U.S. Pat. No. 5,696,662. A number of MEMS variable capacitor device structures have been proposed, including elaborate lateral-motion interdigitated electrode capacitor structures. In the simple vertical motion, parallel plate form of this device, a thin layer of dielectric separating normal metal plates (or a normal metal plate from very heavily doped silicon) is etched out in processing to leave a very narrow gap between the plates. The thin top plate is suspended on four highly compliant thin beams which terminate on posts (regions under which the spacer dielectric has not been removed). The device is ordinarily operated in an evacuated package to allow substantial voltages to be applied across the narrow gap between plates without air breakdown (and to eliminate air effects on motion of the plate and noise). When a dc tuning voltage is applied between the plates, the small electrostatic attractive force, due to the high compliance of the support beams, causes substantial deflection of the movable plate toward the fixed plate or substrate, increasing the capacitance.
Because the change of capacitance, at least in the metal-to-metal plate version of the MEMS variable capacitor, is due entirely to mechanical motion of the plate (as opposed to “instantaneous” electronic motion effects as in varactors or “tunable materials”), the frequency response is limited by the plate mass to far below signal frequencies of interest. Consequently, these MEMS devices will be free of measurable intermodulation or harmonic distortion effects, or other dynamic range problems (up to the point where the combination of bias plus signal voltage across the narrow gap between plates begins to lead to nonlinear current leakage or breakdown effects).
In addition to their freedom from IM/dynamic range problems, normal metal plate MEMS variable capacitor structures offer the potential for substantially lower losses and higher Qs. While the simple parallel plate MEMS structure, which uses the top and bottom plates as the two capacitor electrodes, has a Q problem due to the skin effect resistance, Rac, of the long narrow metal leads down the compliant beams supporting the movable plate, an alternative structure is possible which avoids this problem. If the top (movable) plate is made electrically “floating” (from a signal standpoint; it would still have a dc bias lead on it), and the fixed bottom plate split into two equal parts, these two split plates can be used as the signal leads to the MEMS variable capacitor. (The capacitance value is halved, of course, but the tuning range is preserved.) In this “floating plate” configuration, passage of ac current through the long narrow beam leads is avoided, allowing fairly high values of Q to be achieved, even with normal metal plates.
While this conventional MEMS variable capacitor structure is capable of improved Qs and avoids the intermodulation problems of varactors and “tunable materials,” it has some potential problems of its own. Because only the relatively weak electrostatic attraction between plates is used to drive the plate motion to vary the capacitance, the plate support “spider” structure must be extremely compliant to allow adequate motion with supportable values of bias voltage. A highly compliant suspension of even a small plate mass may render the device subject to microphonics problems (showing up as fluctuations in capacitance induced by mechanical vibrations or environmental noise). Having the electric field which drives the plates directly in the (vacuum) signal dielectric gap may cause another problem. In order to achieve a high tuning range (in this case, the ratio of the capacitance with maximum dc bias applied to that with no dc bias), the ratio of the minimum plate separation to the zero-bias plate separation must be large (e.g., 10× would be desirable). Unfortunately, the minimum gap between the plates (maximum capacitance, and correspondingly, maximum danger of breakdown or “flash-over” failure between the plates) is achieved under exactly the wrong bias conditions: when the dc bias voltage is at a maximum.
Therefore, there is a need in the art for a new tunable filters incorporating MEMS-like HTS variable capacitor structure which offers extremely high Q wide tuning range, freedom from microphonics and breakdown problems, and fully isolated port tuning.